1. Field of the Invention
This invention relates to systems that measure multi-valued signal electrical or optical eye-diagram waveforms of high-speed data communications devices, sub-components or links such as oscilloscopes, jitter analyzers and bit error rate testers as well as built-in test features in ATE equipment and communications devices and systems.
2. Description of the Related Art
Eye diagrams, the bit-synchronized persistent overlay of all possible bit transitions in a multi-valued data stream, are commonly used to grade the quality of a data stream used, for example, in digital data communications. Eye diagrams may be constructed using well understood and generically applied oscilloscope devices and oscilloscope sampling features built into other analyzers including jitter analyzers. Additionally, the ability to draw eye diagrams using a binary decision circuit and a window decision circuit has been previously disclosed by the instant inventor, for example, in U.S. patent application Ser. Nos. 09/541,970 and 10/099,487 which are hereby incorporated in their entirely herein.
Some eye diagrams have poor eye openings which are defined as having a small opening at the center of the eye between logical one values and logical zero values; thereby, making a logical decision of a high level or a low level very error-prone. Communication signals with such eye diagrams are more difficult to use for digital communications.
There are many reasons that bit transitions in a multi-valued data stream may stray from their nominal path from bit to bit down the data stream. These reasons include, but are not limited to, baseline wander of logic zeros and logic ones, noise added throughout the signal and jitter in data bit transitions. Some of these reasons depend on the data value and recent data value history that is being transmitted, where other reasons are completely independent of the data values and recent history.
Conventional techniques have used eye diagram sampling oscilloscopes to draw an eye diagram where the non-data dependent effects were removed by averaging a number of samples of the multi-valued signal amplitude and including in the average only points that corresponded to a particular bit offset (e.g. trigger offset) within the test pattern being used. This has been implemented as turn-key features in sampling oscilloscopes from Hewlett-Packard, Agilent Technologies and Tektronix, and has been generically implemented in research & development laboratories in more manual test setups.
In the aforementioned techniques oscilloscopes are necessary to make such a measurement which, in cases where an oscilloscope is otherwise not required, means the additional expense of an oscilloscope when eye diagrams are desired that remove the non-data dependent effects. An additional drawback of the aforementioned and analogous techniques is that the amount of samples included in the average for individual bit positions of the multi-valued data signal under test is fixed at an inherent effective sampling speed of below 200 K sample/sec and specifically does not scale as with the data rate.
In systems disclosed in the field by the present inventor, eye diagrams are measured and displayed that use either single threshold slice-mode comparators or window-mode comparators. Implementations of these concepts result in eye diagrams with the same features as regular sampling oscilloscopes without emphasizing data-dependent effects.
As disclosed, for example, in Jungerman, et. al. U.S. Pat. No. 6,715,112 triggered waveforms can be performed by sweeping bit error rate measurements versus voltage threshold and differentiating the results to find the peak of bit error rate and to use the threshold voltage of the maximum bit error rate value as the voltage value of the triggered waveform at time various increments. Triggered waveforms such as those disclosed by Jungerman are not eye diagrams. They do not overlay all types of bit transitions in the multi-valued signal atop each other to display the eye opening. Furthermore, restricting a device to operate with an “error performance analyzer” as described in Jungerman, et al. and in conditions where a bit error rate can be measured (e.g. where a second reference copy of the pattern being used in the test is known and synchronized-to) is not a desirable constraint and a method that makes no such restriction would clearly be a substantial improvement.